Effects of radiation of fine vasculature may determine the response of dependent tissues late after a course of radiotherapy. Additional stress to irradiated tissues may cause injury to fine vasculature leading to degeneration and necrosis. It is difficult to study radiation dose response and repair capability of nonproliferating or slowly proliferating mammalian tissues. However, knowledge about radiation repair by normal tissues becomes critical when considering use of radiotherapy techniques which may alter this capability. The primary objective of this research is to determine single-dose and split-dose radiation survival curves for endothelial cells. Injury to the cornea of the eye will cause proliferation of capillary endothelial cells at the limbus. Irradiation prior to and following induction of capillary proliferation permits evaluation of response of both slowly and rapidly proliferating capillary endothelial cells. Morphometric analysis of capillary volume has permitted dose-response assays. The "DO" values obtained were 376 rads for 60Co gamma radiation, 314 rads for neutrons and 214 rads for negative pi mesons. The RBE's determined by comparing doses required to reduce capillary volume to 1% of control values were 1.96 for neutrons and 1.55 for pions. BIBLIOGRAPHIC REFERENCES: Fike, J.R., E.L. Gillette and D.J. Clow: 1976. Response of endothelial cells to neutrons and x radiation. Rad. Res., 67: 639. Fike, J.R. and E.L. Gillette: 1976. Response of capillary endothelial cells after beta, x and neutron irradiation. Int. J. Radiation Oncology, Biology and Physics. Suppl. No. 1, p. 110.